Removal of an unusual hum

Hi. I have an old recording with an unusual effect that I’d like to try to remove. My understanding is that this was done with a cassette recorder with a poor mic and poor SNR, but the biggest problem is this. There is an unusual 60 Hz hum. What it looks like to me is that every time the 60 Hz AC power had a zero crossing, for about 270us around that zero crossing, the microphone audio signal was replace with the AC zero-crossing. So every 8 ms, you get a steep line, alternating in direction each time. Normal Noise Removal does a poor job of removing this, since it removes lots of the signal that I don’t want removed. Here’s how I think it could be removed:

  1. Start with one of these zero crossings. This may have to be set manually the first time.
  2. Select a region from 6 samples before and after the zero crossing, and apply the repair effect.
  3. Skip 1/120th of a second ahead and locate the nearest zero crossing with the opposite slope to the last one, and go back to step 2.

Is this sort of procedure something that could be programmed with a nyquist plugin? Where would I start? Is there, per chance, anything that would do this already?


Post a sample of the work.